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If you have a good copy of the 100-400, then you should get detail you
wouldn't without the TC. The images at 100% will look a little softer, but
the subject displayed at the same size will be better with the TC.

I'd save myself $120 and buy the Kenko Pro 300, though. It does everything
the Canon does, and some think it's even slightly better. It has no
protrusion on the front, either, so you can use it with any lens (good for
sharp macros for extra magnification).

John Sheehy wrote:
I don't even like the AF of the 100-400 without a TC; I can't imagine
relying on it *with* a TC.

Some additional notes:
The 100-400 has no sealed rear end, so as you zoom the lens,
it pumps air into and out of the lens and camera, and
along with it, dust. I have a 100-400, and do not
like it with digital because of the dust factor.

Next, some copies of the 100-400 are not sharp. Mine is not
and produces soft images at 400. This has been noted by pro
bird photographer Art Morris in his newsletters too.
So if you want on, be sure to test the specific lens you will
buy.

If buying a new lens for wildlife, I would recommend
the 300 f/4 L IS or 400 f/5.6 L (no IS) over the 100-400.
Zoom is nice, but not at the expense of performance,
including AF speed, and sharpness.
The AF switch has been off for a long time on my 100-400.

Learn to focus manually, IMO. There are too many wrong things for AF to
focus in in telephoto work, unless you are shooting things that are
isolated from the background.

There is nothing more frustrating than AF that was just spot-on hunting the
wrong way or focusing on the wrong thing while your photo-op ends.

I disagree with this, unless you only shoot static subjects.
One of the great things about IS telephoto lenses is photographing
moving subjects, and it is really difficult for most people
to track focus while panning to follow action. Modern film and
digital SLRs (for the last 15+ years) have predictive autofocus.
That means the camera tracks focus of a subject moving toward or away
from you and when you press the shutter button, the camera sets
the focal point for the time the shutter actually opens, taking into
account the shutter lag (e.g. the time to raise the mirror).

To best use this mode, try a single autofocus sensor and keep the
sensor on the subject's eyes.